1. Field of Invention
This invention relates to a surface acoustic wave element, a surface acoustic wave device, a surface acoustic wave duplexer and a method of manufacturing the surface acoustic wave element.
2. Description of Related Art
Recently, instead of a filter and a resonator using dielectrics, a surface acoustic wave filter and a surface acoustic wave resonator, which are kinds of surface acoustic wave devices, are often used in a high frequency circuit for mobile communication devices, such as cellular phones and cordless phones. This is because the surface acoustic wave filter is smaller than a dielectric filter and has higher electric characteristics in the same size.
Such surface acoustic devices mainly have a piezoelectric substrate, an inter digital transducer (hereafter referred to as “IDT”) obtained by forming electrode films laminated on the piezoelectric substrate, and a package that accommodates the piezoelectric substrate and the IDT. Lithium niobate (LiNbO3), lithium tantalate (LiTaO3) and crystal are used as the material for the piezoelectric substrate. When producing a piezoelectric substrate for an RF band filter, lithium niobate and lithium tantalite, which have a high electric-mechanical coupling coefficient are often used. Because the lithium niobate has a large electric-mechanical coupling coefficient, one having a 64°-rotated Y-cut is used. In addition, because lithium tantalate has a high electric-mechanical coupling coefficient and a relatively small frequency-temperature coefficient, one having a 34-44°-rotated Y-cut is used. Furthermore, in many cases, aluminum having a characteristic such as low specific weight and low resistance, is used.
As described above, surface acoustic wave devices are often used for the RF band (800 MHz-2 GHz), such as a cellular phone. For example, when used in the 1 GHz band, it is necessary to reduce electrode finger width (line width) and electrode spacing of the IDTs to about 1 μm. However, repeated stresses that are proportional to the frequencies are applied to the IDT on the piezoelectric substrate, and migration of aluminum atoms originated by the repeated stress are generated, causing defects, such as hillocks (protrusions) or voids (depletions) on the IDT, which are problematic in that the life of the device is shortened. That is, electric power proofness of the IDT (i.e., the electrode film) becomes an important factor for the life of the device. In addition, deterioration of the IDT noticeably appears not only at high frequencies, but also with the increase in the applied voltage. Moreover, as the frequencies become higher due to the design, the electrode film must be made thinner, and the electrode width must be narrowed. As a result, such deterioration increases.
For miniaturization of cellular telephones, it is effective to miniaturize a splitter (duplexer), which uses a large space in the high frequency part of the cellular phone. Therefore, it has been proposed to change a dielectric duplexer that has been used as the splitter to a surface acoustic wave duplexer. However, although the surface acoustic wave duplexer is a very small component, there is a problem in its electric power proofness against large power applied especially to the transmitter side of the duplexer. The electric power proofness can be improved by reducing the effective power density, which can be done by designing the IDT formation area of the surface acoustic wave device to be large. However, there could be a problem with this technique in that the device cannot be made smaller than a certain size.
For the above reasons, there has been a strong demand to improve the electric power proofness of the IDT of surface acoustic wave devices, that is, the electric power proofness of the electrode film formed on the piezoelectric substrate.
As a means for improving the deterioration of the electrode film due to migration of aluminum atoms, a technique to make the electrode film by an Al—Cu alloy by adding various metals, such as Cu, into aluminum is disclosed at pages 9-15 of J. I. Latham, Thin Solid Film, 64 (1979). By making the electrode film of an Al alloy, because generation of hillock or void on the electrode film is reduced, the electric power proofness of the IDT increases. Techniques to make the electrode film of an Al alloy are disclosed by Japanese Patent Publication No. H7-107967, Japanese Patent No. 2555072, Japanese Laid-Open Patent Application No. S64-80113, Japanese Laid-Open Patent Application No. H1-128607, and the like. In each of the techniques, the deterioration of the IDT is reduced by adding a small amount of various metals into aluminum, that is, the material for the electrode film to control the migration of aluminum atoms.
In addition, diffusion of aluminum is faster at the grain boundary than inside the grain. That is, it is considered that controlling the diffusion at the grain boundary has priority. Therefore, it is thought that the migration of aluminum atoms that originates in the repeated stress in the surface acoustic wave device is caused mainly at the grain boundary. This has conventionally been addressed.
It has been predicted that the electric power proofness can be considerably improved by reducing or eliminating the grain boundary of the aluminum electrode film and more preferably by making the aluminum electrode film a monocrystal. Techniques that make the electrode film substantially a monocrystal are disclosed in Japanese Laid-Open Patent Application No. S55-49014. In the techniques disclosed in Japanese Laid-Open Patent Application No. S55-49014, the performance of the device is improved regardless of the material that composes the surface acoustic wave device. In addition, the techniques to use a monocrystal aluminum film or an aluminum film in which crystal orientation is oriented in a constant direction in the electrode film of the surface acoustic wave device, are disclosed in Japanese Patent No. 2545983. Here, a Y-cut crystal substrate in the range from 25°-rotated Y-cut to 39°-rotated Y-cut is used as the piezoelectric substrate, and by vapor-deposition at high speed (film formation speed: 4 nm/sec) and low temperature (substrate temperature: 80° C.), a (311) oriented film is obtained. This film is said to be an epitaxial film close to a monocrystal. In addition, Japanese Patent No. 2545983 discloses that the use of additives, such-as Cu, Ti, Ni, Mg and Pd, that are considered to be a counter-measure for migration in aluminum has a life-prolonging effect. However, it is not disclosed how such additives are included in aluminum whose orientation is arranged in a constant direction of the crystal orientation.
Japanese Laid-Open Patent Application No. H6-132777 discloses a technique related to an aluminum monocrystal electrode film different from that disclosed in Japanese Laid-Open Application No. S55-49014. In this reference, it is disclosed that an aluminum monocrystal film is obtained by forming the film at extremely low speed. More specifically, it is possible to form an aluminum monocrystal film by a vacuum vapor depositing method on a 128°-rotated Y-cut lithium niobate substrate and to form an aluminum monocrystal film by a vacuum vapor depositing method on a 12°-rotated X-cut lithium tantalate substrate. However, with the above method, because it is necessary to set the film forming speed extremely slow, there is a problem in terms of mass-producability. In this conventional technique, although it is indicated that approximately 0.1-2 wt % of Cu can be added as an additive, it is not discussed that the characteristics of the aluminum monocrystal film are changed by this additive.
International Publication No. WO 00/74235 and Japanese Laid-Open Patent Application No. 2003-101372 disclose that a 38-44°-rotated Y-cut lithium tantalate monocrystal substrate is used as the piezoelectric substrate, an alternate finger-type electrode includes a titanium base metal film and an aluminum film formed thereon, and the aluminum film becomes a monocrystal film due to an effect of the titanium base metal film.
Recently, with respect to the mounting of the surface acoustic wave device, flip chip mounting has been used in which the space for mounting the surface acoustic device is smaller, and the characteristics are more stable than the conventional wire bonding mounting. There are mainly two methods for flip chip mounting. The first is a method that uses solder bumps and mounts the surface acoustic wave device by melting the solder bumps, and the second is a method that uses gold bumps and mounts the surface acoustic wave device by ultrasonic connection. Details of the ultrasonic technique are described below. Bumps of solder, gold or the like are formed on an Al pad electrode of the surface acoustic wave element. With the surface on which the bumps are formed and a printed wiring board facing each other, the bumps of the element and an Au electrode terminal of the printed wiring board are contacted. With such a contact, the bumps and the electrode terminal are connected by vibrating the surface acoustic wave element with ultrasonic waves, and a surface acoustic wave device with strong connections and reliable conduction is manufactured.